Laminated films

ABSTRACT

A cross-laminate of two or more individually stretch oriented films where the directions of orientation of the films are in criss-crossing relation, has a pattern of generally parallel elongated ribs extending lengthwise of the laminate. The ribs are defined between generally transversely coincident convex and concave curved surfaces on opposite faces of the laminate, the concave surface having a radius of curvature greater than that of the convex surface so that the ribs have a thickness greater than the average thickness of the laminate. The laminate material adjacent to lateral boundaries of the ribs is in a tensionless state reversely curved relative to the rib curvature making the material between adjacent pairs of ribs generally flat. The rib pattern is preferably regular with a preferred average transverse spacing between adjacent ribs, measured peak to peak, of about 1-10 mm. The convex surfaces of the ribs can all be on the same side of the laminate or can alternate in series between opposite sides. The pattern of elongated ribs can be interrupted at longitudinally spaced loci to enhance the flexibility of the laminate and the laminate can have an undulating curvature when viewed from the side. A process and apparatus for making the ribbed cross-laminate by means of intermeshing grooved rollers are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns an improved cross-laminate of oriented films ofthe general type defined in the preamble of claim 1 and improvedprocesses and apparatus for manufacturing cross-laminates of thisgeneral type.

2. Description of Related Art

Processes for production cross-laminates of oriented films are known inparticular from GB-A-1,526,722. In this known invention, the material ismelt oriented in a generally uniaxial manner before cross lamination,and is biaxially oriented after cross lamination, preferably near roomtemperature. The melt orientation may be very weak, but is alwayscombined with the use of blends of polymers, which are sufficientlyincompatible for formation of a two- or multiphase grain of polymerunder the influence of the melt-orientation, which grain has a verysignificant influence on the strength properties of the finalcross-laminate. In order to enhance the tear-propagation resistance thebonding between the films is a generally weak bonding, but may besupplemented with a strong bonding in spots or lines.

The objective of this known invention is to provide for a film materialwhich exhibits high strength properties in all respects. One of itsimportant uses is for bags and similar packaging uses.

Further according to the above mentioned British Patent, the transversestretching which is subsequent to the cross-sandwiching of themelt-oriented films (see the introduction to claims 12 and 24) ispreferably carried out by passing the sandwich through several sets ofmutually intermeshing grooved rollers, the grooves of which are as fineas practically possible. Also, the sandwich is normally (but notnecessarily) stretched longitudinally in continuous manner betweensmooth rollers before, between or after said steps of grooved rollersstretching. (I distinguish between "sandwiching" which may involve, butneeds not involve a bonding of the films to one another, and"laminating" which always involves such bonding). For obtaining optimumenergy-absorbing properties (such as e.g. shock-tear propagationresistance) the different stretching steps, following thecross-sandwiching, are preferably carried out at temperatures very muchbelow the melting ranges of the films, and may even be carried out atnormal room temperature.

Further according to the above mentioned British Patent, thecross-lamination of films having a uniaxial or an unbalanced biaxialmelt-orientation can be carried out already in the extrusion processunder use of counterrotating dieparts, but can also be established onthe basis of helical cutting of melt-oriented, tubular films. Thus thetubular films can be melt-oriented mainly in their longitudinaldirection, helically cut e.g. under 45 degrees after solidification, andsubsequently sandwiched in such manner that the said main directionscriss-cross one another (i.e. become perpendicular to one another if thecutting angles all have been 45 degr.). In this connection, recent WIPOpublication WO-A-89/12533 discloses particularly practical methods forspiral cutting of tubular film, and also discloses a suitable method toachieve a melt-orientation, which, if desired, can be perpendicular tothe machine direction (i.e. the continous direction) of the film. Thelast mentioned method consists in first hauling-off the tubular filmfrom the extrusion die in a screw-movement to give the tubular film amelt-orientation which forms an angle (e.g. 30 degrees) to the axis ofthe tube, and then helically cutting the tubular film (e.g. under 60degrees) in the way which increases the angle between the machinedirection and main direction of melt-orientation. Thus, using the abovementioned example that the "screwing" is under 30 degr. and the cuttingunder 60 degr., the melt orientation will become perpendicular to themachine direction after the helical cutting. This film can continuouslybe sandwiched with a film which is melt-oriented mainly in itslongitudinal direction (the machine direction) to form a perpendicularcriss-cross arrangement.

One of the features mentioned in the introduction to the present claim 1is that the cross-laminate exhibits a pattern of striations constitutedby thickness variations. Such pattern will always be formed as a resultof the stretching between grooved rollers--except if special precautionsare taken, which will be discussed later. Following the teaching of theabove mentioned British Patent (see in particular FIGS. 8 and 9) thesethickness variations which form a longitudinally striated pattern, willoccur at random as a result of interference between the stretchingpatterns in each step of the grooved roller stretching. When notexaggerated, the striated pattern can have a positive influence on thetear-propagation properties and some positive influence on theself-supporting capability (the stiffness when the material is bent overa line perpendicular to its continuous direction). However, if thisrandom pattern of thickness variations is very pronounced, it has a verynegative influence on u-v stability, printability and the resistance topenetration by humidity, aroma substances and vapours.

As a special feature of the technology, the individual films can beblocked together by the transverse stretching together between groovedrollers, and this effect can be controlled by suitable surface layers onthe films, which therefore originally are produced by coextrusion. Inthe coextrusion process, there is also made provisions to give the finallaminate thin surface layers of desirable properties, in particularlayers to improve heat-sealing or layers to control the frictionalproperties.

Further developments of the known technology referred to above aredisclosed in U.S. Pat. No. 4,629,525. This describes a stabilizationprocess, in which a cross-laminate of the above mentioned kind is heatedwhile allowing at least 7% transverse shrinkage (i.e. transverse of thecontinuous direction of the laminate and of the striations produced bythe grooved rollers) and preferably also a longitudinal shrinkage.Besides the stabilizing effect, which means that the coldstretchedlaminate does not tend to shrink further during use or storage at normaltemperatures, there are important side-effects. One is that the abovementioned thickness variations (the striation effect) can becomesignificantly reduced, since the transverse shrinkage mainly takes placewhere the material is transversely over-stretched. Another importantby-effect is a significant increase of the yield point in the transversedirection. A third effect is increase of the weak bonding whichoriginally is produced by the blocking-together of the spiral cut filmsbetween the grooved rollers.

The transverse contraction is preferably obtained by feeding thecross-laminate, while it is pleated to a suitable extend onto a heatedroller (from where it may continue over more heated rollers) so that thepleating gradually disappears while the cross-laminate contracts.

This patent also discloses advantageous polymer blends for the mainlayer of the coextruded film for this general type of cross-laminates,in particular blends of high-molecular-weight-high density polyethyleneand linear-low density or linear-low-density-like polyethylene atsignificantly lower molecular weight than the first mentioned component,to which optionally may be added polypropylene (as these cross-laminatesare more precisely defined in claims 25-29 of the above mentionedpatent). Finally, the said patent specification discloses that crosslaminates for manufacture of sacks preferably should be made frommelt-oriented tubular film cut under an angle between 10° and 35°instead of 45°. Further improvements in the general type ofcross-lamination technology described above is disclosed inWO-A-88/05378. Here, at least the first pair of grooved rollers is ofspecial construction and function. The grooved, fine, circular "teeth"have inclined sidewalls, the sidewalls on the cooperating groovedrollers match very exactly, and they operate under a high roller'spressure so that the transverse stretching takes place not only bytentering but also by squeezing or "lateral calendering" of the laminateor sandwich (all as further described in that citation).

By this method it has been possible to manufacture above describedcross-laminates in improved quality and at highly increased productioncapacity. This increase is made possible because two or more crosslaminates can be produced together in this process, and separated fromone another at the end of the manufacturing process.

The inventor has also combined into the technology the embossmentlocalised adjacent to heat-seals in a bag, which is described inWO-A-89/10312 and which is adapted to produce a shock-absorbing orforce-controlling effect thereby improving the drop strength of aheat-sealed bag of oriented or rigid film material and filled withpowder or granulated goods.

By a combination of the above mentioned inventions the inventor has beenable to manufacture in a commercial and economically feasible process,heat-sealed heavy-duty bags of cross-laminates in gauge e.g. 60-80 gm⁻²,which with respect to yield point tensile strength, puncture resistance,tear propagation resistance and drop-performance have proven superior tobags from low-density or linear-low-density polyethylene of the doublegauge. However, due to lack of self-supporting capability of thecross-laminate in such gauges ("flimsiness") the bags have not yet metgeneral market acceptance, since the automatic or manual handling inconnection with the filling (the "bagging") has been considered toodifficult or unreliable. In this connection it is noted that theself-supporting capability (which is a result of the stiffness of thefilm) of a film of even thickness varies with the second power of itsthickness.

OBJECTS OF THE INVENTION

One major objective of the present invention, therefore is to providefor a cross-laminate (of the above described type) having significantlyimproved self-supporting capability. Other objectives, will appear fromthe description below.

SUMMARY OF THE INVENTION

The special features of the product according to a first aspect of thepresent invention appear from the characterising part of claim 1. Thespecial curved form of the thicker ribs with immediately adjacentmaterial, namely bending to one side in the main body of the rib, andoppositely near its boundaries, provides for high stiffness when thecross-laminate is bent around a transverse line, and the lowerthicknesses between the ribs facilitates the bending around alongitudinal line, which also is important in manual or automatichandling of the cross-laminate, since the necessary self-supportingcapability of a sheet-material often requires that the operator or thesheet-handling machine gives the sheet a slight bending during theoperation.

The ribs are preferably arranged in a generally regular pattern oversubstantially the entire width of the sheet. It should be noted however,that the process (which will be described later) can be quite sensitiveto the influence of the different process parameters, a reason why therecan be quite remarkable deviations from the regularity of thedistribution and also of the rib-form.

Depending on the intended use of the cross-laminate, the curved ribs caneither, in an alternating arrangement, protrude from both surfaces ofthe cross laminate (see FIG. 2) or protrude from one surface only (seeFIG. 1).

A second aspect of the invention concerns an improvement of the abovementioned stretching method described in WO-A-89/10312, whichimprovement in particular is useful in connection with efficientmanufacture of the curved rib structure defined in claim 1, but alsofinds other uses due to a high regularity of stretching which isachieved.

According to this second aspect of the invention there is provided amethod in which continuous polymeric sheet material is biaxiallystretched by a process comprising the steps of:

1) combined transverse tentering and transverse squeezing by action ofcompressionally working grooved rollers,

2) longitudinal stretching between rollers,

3) a second forming and transverse stretching between grooved rollers.

This aspect will be further described after the general description ofthe embodiments of the curved rib structure and of the method formanufacturing this structure.

Now returning to the first aspect of the invention, the laminate withcurved ribs, the average division between neighbour ribs preferably isbetween 1 and 10 mm, measured from peak to peak and taken as an average.

The average thickness of material between the boundaries of the ribs,defined as the locations where the rib thickness meets the average ofthe cross laminate in a local region around and including the rib andseveral neighbour ribs is at least 15% and preferably at least 30% lowerthan the maximum thickness of the rib.

Thickness at a given point of the laminate surface is understood as theshortest distance from that point to the opposite surface. In the ribs,this is normally not the distance along the route perpendicular to thesurface in the given point, because the two surfaces are not parallel.

The average thickness of the material calculated by measuring the weightof a specified area of material (in the tensionless stage) and from aknowledge of the average density of the polymer material used to makethe laminate. The average thickness is thus in the direction ofperpendicular to the overall plane of the laminate.

Comparing the thickness within the ribs with the average thickness ofthe material, it must be taken into account that there normally is aconsiderable variability in thickness of extruded film material(normally not less than ±5% and often ±10%), that the grooved rollersdue to bending can produce a lower degree of stretching at the middle,compared to that of the edges, and that on the other hand, the spreadingaction which follows the grooved rollers stretching and which normallyis carried out with banana rollers, normally is most efficient at themiddle. Therefore the mentioned comparison must be on a local basis andnot a comparison with the average thickness of the entirecross-laminate.

Generally speaking it is found that the thickness variations which areinherent in this first aspect of the present invention, do notsignificantly reduce the strength properties such as ultimate tensilestrength, yield point, tear propagation resistance, impact strength andpuncture resistance. The reason for this is that the lower thickness inportions forming longitudinal lines are compensated by a higher degreeof transverse orientation. However, when the laminate is used to makebags, the drop strength of the bag when filled with powdered orgranulated goods and heat-sealed, can be significantly negativelyinfluenced by very thin portions. The bag could then rupture along aline immediately adjacent to the heatseal, in which line the materialwas melted during the heatsealing and thereby lost all or most of theorientation. In a preferred embodiment of the invention therefore,generally substantially no local thickness is lower than 30% andpreferably no lower than 50% of the average laminate thickness.

The angle between two tangency planes on the concave side of the ribnear its two boundaries where this angle is maximum--namely the angle vin FIG. 3--should preferably be at least 10 degrees taken as an averagefor the different ribs and more preferably between 25 and 90 degrees.

This is of importance for the self-supporting capability, but also togive the cross-laminate more bulk and to give it a resilient characterwhen it comes under vertical pressure. Such bulk and resilience is ofimportance for the handling of the cross-laminate and is also anobjective of this first aspect of the present invention.

The requirement that the surfaces of the material in or adjacent to theboundaries of the ribs in the tensionless state are bent in the oppositedirection to the respective surface in the rib results in thecross-laminate having a generally flat appearance rather than agenerally corrugated appearance. For instance in the cross-laminate thatsurface of the rib which is convex within the rib itself becomes concaveat or near (usually just beyond) the boundary of the rib. It may remainsubstantially concave until near or at the boundary of the adjacent rib,for instance where the two ribs are on the same surface of thecross-laminate. The intention is that near to the boundary of the ribthe curvature of the surface becomes concave where it was convex andvice versa and the degree of curvature is relatively high or sharp (i.e.the radius of curvature is small) near to the boundary and thendecreases i.e. flattens out, before increasing again towards the nextrib. The shape of the surface of the embodiment with adjacent ribsformed on the same side of the cross-laminate can be thought of ashaving a wave-like form with the thickened rib portions having a profilesimilar to the positive (or the negative, as the case may be) portion ofthe sine function but with the portion between the ribs being relativelystretched out or expanded (along the abscissa). For a cross laminatewith ribs on alternating surfaces the profile is again wave-like withthe ribs having a profile similar to the sine function (positionednegative peaks) but again separated by stretched out sections (along theabscissa).

The cross-laminate of the present invention thus differs from theproduct disclosed in WO-A-8805378 and illustrated in FIG. 6 thereof. Thelaminate shown in FIG. 6 does not have thickened rib sections. Also thesurfaces of each side of the cross laminate are generally sinusoidal inprofile and do not have the stretched portions between the maximum andminimum regions.

In the foregoing, the first aspect of the invention has been describedwith a view to the application for bags and similar applications, inwhich high stiffness in one direction and much lower stiffness in theother direction is desirable. However, in an embodiment of this aspectthe bigger thickness of the ribs and their u-like profile is eliminated,at least in part, in a pattern of transverse lines. These lines act as"flex-lines" so that the cross-laminate readily can be bent, not onlyover a longitudinal, but also over a transverse line. In this embodimentthe objective is the introduction of bulk and vertical resilience in thecross-laminate, at the same time as this is given a high flexibility,which is desirable mainly in textile-like applications, e.g. fortarpaulins and coversheets.

The elimination of thickness and u-shape can be carried out byembossment at a temperature below the melting point, care being takennot to ruin the transverse tear properties, or by a stretching localisedto the said transverse lines, like the longitudinal stretching in U.S.Pat. No. 4,285,100 (Schwartz).

As a special feature of this embodiment the production of thesetransverse "flex-lines" can be carried out at a temperature which, incombination with the applied pressure, produces a strong bonding,preferably a true welding in these lines, while the rest of thecross-laminate is kept weakly bonded or not bonded at all. While agenerally weak bonding is necessary for the tear-propagation resistance(as already mentioned in the introduction) the strong bonding localisedto the "flex-lines" has the effect that the cross-laminate can berepeatedly and strongly bent in both directions, as when a tarpaulinflip-flops in a strong air jet, without delaminating. In fact theapplication of transverse "flex-lines", which at the same time are linesof strong bonding, is very useful not only for cross-laminates withu-shaped ribs, but also for any cross-laminate of the type defined inthe introduction to claim 1.

As mentioned above in the description connected with claim 1, it isoften advantageous that the laminate shows big resistance to bendingaround longitudinal lines, but low resistance to bending alongtransverse lines. However, there are also applications in which a highresistance to bending is required in all directions, e.g. in the usualmanufacturing process for glued block-bottom bags. For suchapplications, the laminate with longitudinal ribs as described shouldpreferably exhibit the additional feature that it is undulated orzig-zagging when viewed in its longitudinal sections.

This can be achieved in simple manner, e.g. in the bag making line, bypassing the laminate with the described rib structure first through afirst set of nip-rollers of which one is a rubber roller and the other agear roller, which may be relatively sharp edged on the tips of the gearteeth, operating so that the laminate gets a permanent bend around thesetips, and then through a second similar set of nip rollers set-up tobend the laminate in the opposite direction and so that the laminatealternately is bent in one and the other direction. The longitudinalribs of "U-shape" as described have an important function in thestabilisation of this bending.

Preferred compositions for the cross-laminate of the first aspect of thepresent invention are tri-extruded films with a main layer for strengthin the middle and minor layers at the surfaces for facilitated bondingbetween the films and for heat-seal properties of the cross-laminate andthat the main layer of said films consists of 10-30% low densitypolyethylene mainly of the linear type, and the remainder high-molecularweight polyethylene, high molecular weight polypropylene or acombination of both. These are selected to give a high stiffness inaddition to high strength values and heat-sealability.

The process of the first aspect of the present invention is fortreatment of a cross-sandwich which is stretched in the directiontransverse to the direction of advancement of the web through theprocess using grooved rollers, and in the process the at least two filmsare continuously laminated together: and is characterised in that saidtransverse stretching is effected by forming below the melting point ofthe thermoplastic material an undulated cross-sectional shape on atleast the cross laminate of the cross-sandwich and during or after saidforming process, stabilising the tip portions of the undulations on atleast on one side of the cross-laminate and transversely stretching thecross-sandwich between the stabilised portions by the use of groovedrollers, this stretching being adapted to maintain the shape or a memoryof the shape of the material in the stabilised portions of the undulatedshape and subsequently heat-treating the cross-sandwich so that thematerial between the stabilised portions shrinks along a hypotheticalplane lying substantially midway between the surfaces of thecross-sandwich in the local region of shrinkage in a direction lyingperpendicular to the direction of movement of the web and if necessary,so that any stabilised portions of the undulating memory of which hasbeen retained in the grooved roller transverse stretching operation arereshaped at least in part, thereby to create a rib patternedcross-laminate in claim 1 with thickened rib portions as defined inclaim 1.

Most practically, the undulation of the cross-sandwich is carried outunder use of grooved rollers. The simplest, but not most efficient wayof giving the tip-portions of the undulated structure a stabilisation ofthe curved form is by carrying out this forming close to the meltingpoint of the material. The material is first heated to such temperatureand is then formed over one or between two slightly intermeshing groovedrollers, the temperature of which is kept well below the temperature ofthe cross-sandwich so as to avoid that the material stretches on thetips of the grooved roller or rollers. The cross-sandwich is cooled downbefore the further processing.

An alternative way of carrying out the stabilisation is by cross-linkingunder use of irradiation. The curved portions to become stabilised areirradiated, e.g. with accelerated electrons while the rest of thecross-laminate is not irradiated. A suitable cross-linking agent may beadded in the extrusion process. This may give a very efficientstabilisation but is also relatively complicated in practicalproduction.

The most practical and efficient way of carrying out the forming andstabilisation of the curved portions, is to carry out these two actionsas one process by use of compressionally working grooved rollers. Thismeans grooved rollers, in which the grooves have outwardly inclined sidewalls of which each has a portion that matches with (i.e. during theoperation is substantially parallel to) a similar portion on theopposite grooved roller, and which grooved rollers are operated under ahigh roller pressure to squeeze the material between these parallelportions. Reference is here made to WO-A-88/05378, which already hasbeen mentioned in the description of the background of the presentinvention, see e.g. FIG. 2 of said publication. For use in the firstaspect of the present invention, stretching of the portions on the tipsof the grooved rollers should be avoided or minimised at least on oneside of the cross-laminate, so that these portions become thicker thanthe portions which have been squeezed.

The conditions can with advantage be adapted even to increase thethickness of the cross-sandwich portions on the tips of the rollergrooves.

It was found that the forming under use of compressional forces has anadequate stabilising effect.

There can accidentally occur a slight amount of stretching at the verymiddle of a groove tip during compressional stretching, resulting in anarrow thinner portion at the middle of the generally u-shaped rib, inwhich thin portion the otherwise convex surface may unintendedly becomeconcave (see FIG. 4). As long as this phenomenon does not disturb thegeneral character of the ribs, the product and method will still fallunder the scope of the invention.

In order to enable stretching of wide film under use of the highpressures for the squeezing and under maintenance of a high precision inthe fitting of the grooved roller surfaces into each other, thestretching arrangement is preferably divided into segments over thewidth of the cross-sandwich as disclosed in detail in the abovementioned WO-A-8805378, see FIGS. 3, 4 and 5.

As already discussed above 88/05378 discloses that what is intended tobecome two or more cross-laminates can be transversely stretchedtogether, one on top of the other, between the compressionally workinggrooved rollers, and then separated later. This procedure is particularadvantageous in connection with the first aspect of the invention notonly because of the higher production capacity and improved quality ofthe surfaces which are separated from each other, but also because thesqueezing of material into the portions on the tips of the groovedrollers is facilitated when the cross-sandwich is relatively thickduring this operation.

The material needs not be longitudinally stretched after being arrangedas a cross-sandwich, however in most cases the cross-sandwich islongitudinally stretched prior to or as a step which is second to thestabilisation step, no matter by which means this stabilisation iscarried out.

A particularly efficient and suitable way of producing the "U-rib"structure combines the second aspect of the present invention into thisfirst aspect by starting the treatment of the cross-sandwich with afirst compressional grooved roller stretching, then the cross-sandwichis stretched longitudinally, then again treated between compressionallyworking grooved rollers, while this second compressional grooved rollerprocess is adapted to fit with the undulations produced by the firstcompressional stretching so that the action of the second compressionalgrooved roller stretching increases the curvature and stabilisation ofthe material in the rib-portions which was started between the firstcompressionally working grooved rollers. This "registration" of the twoprocesses does not present any particular problem, provided all threeabove mentioned stretching steps are carried out in-line with thecross-sandwich passing from one roller to the next with only shortdistances between said rollers.

During the longitudinal stretching over rollers positioned close to oneanother, the undulations will partly disappear due to tendency toshrinkage of the polymer material in the transverse direction duringstretching in its longitudinal direction, but there will always be atleast traces of the undulation left, and this facilitates the"registration" of the second grooved roller process.

Further details are disclosed below in connection with description ofthe second aspect of the invention.

The total longitudinal stretching of the cross-sandwich can withadvantage be divided on two steps, one as described above, and thesecond following directly after the second step of compressionaltreatment between grooved rollers. Prior to this second step, thecross-sandwich is preferably cooled in order to preserve the undulationsor the "memory" of the undulations.

The cross-sandwich can proceed directly from the second compressionaltreatment and after a subsequent longitudinal stretching step to aheat-treatment under mild tentering or by ironing, but usually a higherdegree of transverse stretching is desirable before the heat-treatment,and this may be carried out by use of a simple grooved roller stretchingprocess, i.e. such that the cross-sandwich touches the surfaces of thegrooved rollers only on the tips of the grooved surface pattern.

If only one step of compressional transverse stretching has been applied(or another process for forming and stabilising the curved portions),such simple grooved roller process has to be used in any case.

As a measure to maintain the memory of curved shape in the stabilisedportions of the cross-sandwich, this stretching is preferably carriedout near room temperature, e.g. between 15°-40° C.

According to a further aspect of the invention there is provided animprovement in a process of transversely stretching a film as describedin WO-A-8805378. The process is for transversely stretching a film orfilm sandwich by passage between intermeshing, driven grooved rollers,which grooves are circular or helical, and in which the film touches thesurfaces of the grooved rollers only on the tips of the grooved surfacepattern, and the improvement is that in order to remove stretching heatand keep the polymer material at the desired temperature during thestretching, a flow of fluid medium, preferably air or water, is directedthrough the nip of the grooved rollers on one or both sides of thepolymer material. It was found that this measure, in addition topreservation of the memory of curved shape, also helps to make thestretching even, and therefore this measure can with advantage beapplied to transverse stretching between grooved rollers of anystretchable polymer film or film sandwich below its melting point, inparticular when the material tends to neck down instead of stretchingevenly. This tendency to necking will be most pronounced when thepolymer material is relatively stiff at the given temperature, and inthe case of cross-sandwiches when the angle between the main directionof orientation and the machine direction is relatively low.

While this aspect of the process is generally applicable to groovedroller stretching, the following again concerns the achievement of thecross-sandwich with u-shaped thicker ribs.

The cross-sandwich will leave the grooved rollers in an undulated formwith an undulation which need not have any relation to the originalformation of curved portions.

The width of the cross-sandwich from edge to edge, measured along theundulated shape, and divided by the straight distance from edge to edge,indicates the average transverse stretch ratio. A part of thisstretching will be eliminated by shrinkage during the heat-treatment,while a part must remain in the final product. The mechanicallydetermined allowances for shrinkage and the temperature of theheat-treatment must be carefully adjusted to allow the straightening outof the material between the stabilised portions, at the same timeavoiding elimination of the curved form in the stabilised portions, orallowing a recovery of this form, if it was lost before.

This controlled transverse shrinkage can be carried out in an oven,while the edges of the cross-sandwich are guided by conveying devices,but more conveniently by feeding the sandwich in an evenly pleated stateto one or more heated rollers, the degree of pleating adapted toestablish, in conjunction with the transverse shrinkage, the needed lowtransverse tension. This is a new use of the method for transverseshrinkage which is described in U.S. Pat. No. 4,629,525 which alreadywas referred to above. By correct adjustment of the degree of pleatingand maintenance of a low longitudinal tension, the system will permitshrinkage and establishment of the needed low tension, and the heatedrollers will perform an ironing effect.

During this treatment, the bonding between the individual films can beincreased to the wanted final value.

Preferably the cross-sandwich is also allowed a longitudinal shrinkagesimultaneously with the transverse shrinkage.

The control of the degree of pleating of the cross-sandwich when itmeets the heated roller or first heated roller can be established bydifferent methods of which the preferred now shall be explained.

The cross-sandwich coming out of a first grooved roller unit istransversely tightened in a tenter-frame or by use of banana-rollers orthe like, and the intermeshing between these grooved roller is adjustedso that the width after the tightening is substantially equal to thedesired final width. In this state, the transversely tightenedcross-sandwich is fed to a final set of intermeshing grooved rollers,which are set to give exactly the desired degree of pleating. Nospreading-out is performed between this last set of grooved rollers andthe first heated roller.

In order to establish the correct conditions for the contraction, thedegree of pleating at the entrance to the heating-treatment preferablyis at such a value that there still are a few narrow pleats in thematerial as this leaves the last heated roller and these pleats are thenremoved by very mild tentering, e.g. with a banana-roller. After theheat-treatment, the cross-sandwich is cooled to ambient temperature. Ifthe process involves separation of the sandwich to severalcross-laminates, this is preferably carried out as a last step.

As it appears from the description of the background of the invention,the known technology to which the invention is connected, starts with agenerally uniaxial melt orientation, which normally is a very weakorientation of polymer blends capable of forming a fibrillar morphology("grain of polymer") in connection with this meltorientation. The scopeof the present invention is neither limited to the use of such blends,nor to the use of weakly melt oriented films for the cross-sandwiching.The tubular films which are uniaxially or unbalancedly biaxiallyoriented and spiral cut, may have received this orientation below themelting point. In this connection it is observed that strongly ormedium-strongly oriented film (uniaxially or unbalancedly biaxiallyoriented) which after cross-lamination are further stretched, usuallythereby receives a very strong tendency to curl up along one of the mainaxes of orientation. However, by use of the first aspect of the presentinvention this tendency is interacted by the u-formed ribs, and a muchstronger uniaxial character of the initial films is possible, when thisis desirable. The achievement of this effect is a further objective ofthe invention.

The following concerns the second aspect of the invention, the improvedstretching method. It should be noted that the second forming andstretching step preferably but not necessarily also is carried outcompressionally by combined transverse tentering and transversesqueezing action. As it appear from the foregoing, this second aspect inparticular is suitable as first step or steps of the manufacture of the"U-rib" laminate, namely for forming and stabilising the curvature.However, the method can with advantage also be used for other purposes.Thus, if subsequent processes are not set-up as explained above formaintaining or for recreating the "U-structure", the final product maybe completely without such curvature, but can exhibit a regularity whichis unusual for sheet material stretched between grooved rollers.

By a subsequent calendering process, which even may be carried out atroom temperature, it is easy to make the thickness perfectly even ifthis is desirable. Examples 2 and 3 will present differentconfigurations of stretched sheet material made under use of this methodand will explain the differences between the process steps used toarrive at these configurations.

It should also be mentioned that, although this stretching method inparticular finds uses in the manufacture of cross laminates, and morespecifically in the manufacture of cross laminates from polyolefins, itis applicable in general to all kind of thermoplastic, stretchable sheetmaterial including sheet materials which are not laminates.

There are two alternatives for coordination or "registration" of the twosteps of compressional transverse stretching and forming, one being toadjust the division between the undulations on the sheet entering thegrooved rollers of the last forming step and the division between thegrooves of these rollers to fit with each other. In this connection itis noted that, due to elastic recovery forces, the undulated sheetleaving the grooved rollers of the first forming step will try to expandwith relatively big force, thereby tending to increase the divisionbetween the undulations. On the other hand the sheet will try tocontract in the transverse direction during the longitudinal stretchingprocess, also with a relatively big force, and thereby will tend toreduce this division. By appropriate choice of process conditions theseopposite tendencies can be brought to outbalance each other, butnormally this will not be a convenient way to make the "registration",because it sets strict limits for the choice of stretching ratios,roller pressures and stretching temperatures. However, if the netexpansion or contractions (as the case may be) is found experimentallyunder the wanted conditions, the grooved rollers of the second formingstep can be constructed with a precalculated division between thegrooves, and an exact fitting between this division and the undulationscan be achieved by small adjustments of the process conditions.

It should be noted that, when the undulations are deep, they show apronounced tendency to "fall into track" on the grooved rollers ofsecond forming step. Therefore, if the division of these grooved rollersand that of the undulation do not exactly fit to each other, there willstill be "registration" at intervals along the width of the sheet andthese intervals may be relatively wide and together cover most of thewidth, but between these intervals of "registration" there will benarrower intervals in which the structure will be irregular.

The alternative way of achieving the "registration" is by using forfirst and second forming process grooved rollers of the same division,while keeping the distance between each set of rollers which the sheetpasses over between the first and second forming processes sufficientlyshort to force the sheet to keep constant the division between theundulations of the sheet. Preferably the forces acting to keep thedivision between the undulations constant are increased by supplyingrollers which the sheet passes over after leaving the grooved rollers ofthe first forming process and before meeting the grooved rollers of thesecond forming process, with guiding tracks, of the same division as thedivision on the grooved rollers for the first and second formingprocess.

If smooth rollers are used between the two forming steps, but stillusing a very short distance between each set of rollers, there mayoccasionally come irregular bands in the sheet, but in general thestructure will become regular.

In a further embodiment of the stretching method the grooves on therollers for first and second forming process are adapted to squeeze thesheet within bands only, whereby the sheet portions to become peaks ofthe undulations are not squeezed, so that the squeezed bands of thefirst forming process are wider than the squeezed bands of the secondforming process.

The avoiding of any squeezing on the sheet portions which become peaksis well known from WO-A-89/10312, and the special feature of thisembodiment is that the first forming process exerts its squeezing actionover wider areas efficiently undulated when it meets the grooved rollersof second forming process and therefore best apt to "fall into track",while on the other hand the narrower areas of squeezing during secondforming step secure that there can remain thicker "ribs" in the finalproduct when this is wanted, e.g. for the "U-rib" structure which isdescribed in detail in this specification.

The sheet coming out from the second forming process may be used as itis for special purposes, but normally further longitudinal and/ortransverse stretching processes are carried out subsequent to thissecond forming process, e.g. for manufacture of the described "U-rib"laminate.

In this connection, an embodiment is characterised in that the sheet isheated prior to the first forming process, kept at substantiallyunchanged temperature before and during the second forming process, andis cooled before the subsequent stretching. This cooling helps topreserve the curvature or memory of curvature as needed in themanufacture of the described "U-rib" laminate. It also enhances tearstrength of the final product and improves the evenness of transversestretching between grooved rollers, when this stretching is followed bya heat treatment under transverse shrinkage (as described in theintroduction to this specification).

Finally an embodiment is characterised in that the sheet is subjected tocalendering between smooth rollers subsequently to the last transversestretching step which is carried out before final use of the sheet.

As already mentioned, the stretching according to this second aspect ofthe invention produces a regularity which is unusually high compared toknown stretching processes under use of grooved rollers, even when a lowstretching temperature is chosen as required for several purposes. Ifthe stretching process according to this second aspect, including thesubsequent stretching steps, do not alone give a fully even sheetthickness and if this required, the evenness is easily achieved by thementioned calendering even when the temperature during the calenderingis at or about room temperature, e.g. up to 50° C.

The invention also comprises apparatus to carry out any of theembodiments of the method according to both first and second aspect. Theconstruction of such apparatus appears from the description of themethod and also from the apparatus drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying drawings in which:

FIG. 1 shows the embodiment of the product in which the u-shaped ribsprotrude from one surface only,

FIG. 2 shows the embodiment in which they protrude from both surfaces,

FIG. 3 shows a rib and adjacent material more in detail,

FIG. 4 shows an irregular rib, still falling under the scope of theinvention and

FIG. 5 show a preferred process line for the "U-rib" structure of thefirst aspect of the invention and for the second aspect of the inventionas a flow-sheet.

FIGS. 6A and 6B shows a schematic representation of apparatus suitablefor carrying out the process of FIG. 5.

FIGS. 7 and 8 are sections through pairs of grooved rollers at theirsurfaces in the nip.

FIG. 9 is a schematic representation of a line suitable for carrying outthe process of the second aspect of the invention.

FIG. 10 is a section through one of rollers of FIG. 9 at its surface.

FIG. 11 is a section through another of the rollers of FIG. 9 at itssurface.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 show the thicker ribs, which have cross-section of formlike a flat U, and the bendings in the opposite direction at or near theboundaries of the ribs. The cross-section of the cross-laminate shown inFIG. 1 will normally be formed when e.g. four films are taken togetherin the "Lateral Calendering" process (i.e. through the compressionallyworking grooved rollers) are separated into two cross-laminates at theend of the process line, provided there is only used one step of"Lateral Calendering". In the more efficient procedure in which twomutually registered "Lateral Calendering" steps are used withlongitudinal stretching in between, the cross-section of FIG. 2 willnormally be formed. This cross-section will also normally be the resultof using one "Lateral Calendering" step alone without any separationfollowing.

FIG. 3 illustrates the definition of the angle (v) which is the anglebetween the two tangency planes on the concave side of the rib near itstwo boundaries at the locations where this angle is maximum. The valueof v is at least 10°, taken as an average for the different ribs acrossthe cross-laminate.

FIG. 4 shows a U-shaped rib which can be formed if a slight amount ofstretching has taken place on the tip of the rib-forming grooved roller.This deviation is acceptable as a compromise to enable the use ofcheaper machinery.

The flow sheet FIG. 5 summarises the most preferable procedure, whichinvolves two steps (steps 4,6) of "Lateral Calendering", and the processsteps to form the ribs of the laminate (steps 9,10) has been describedin detail in the foregoing.

EXAMPLE 1

This example is based on example 3 WO-A-88/05378, with some variationsas explained hereinafter.

The example relates to the manufacture of a high-strength cross-laminatein a commercial process using a technique in which the sheet isstretched and laminated while in double thickness and is separated atthe end of the process. Each layer is a coextruded film and contains

(1) one surface layer which acts as release layer in the manufacturingprocess, and which at the same time improves the heat-sealing properties(in the following referred to as release/seal layer) and

(2) another surface layer which promotes the bonding (blocking) of theplies of the laminate to each other.

A tubular film is extruded comprising a main layer in the middle, inwhich layer the strength essentially resides, and the above mentionedrelease and lamination layers.

The three layers form 75% (main), 15% (release) and 10% (lamination) ofthe total film.

The gauge of the extruded film is 62 g m⁻². The main layer of extrusionconsists of 70% HMHDPE of density 0.95 (melt flow index about 0.05according to ASTM D-1238 condition E) 20% LLDPE of hexene type and meltflow index 0.8 (ASTM D-1238 condition E)and 10% homo-polypropylene ofmelt index 0.3 (ASTM D-1238 condition L). The release/seal layerconsists of 100% LLDPE while the lamination layer consists of anintimate blend of 70% of the same LLDPE+30% EPDM(ethylene-propylene-dimer) melt flow index about 0.4 (ASTM-D1238condition E) of trade name "Nordel NDR 5715". (Nordel is a trade mark).The extrusion temperature is 240° C., the diameter of the annularextrusion orifice is 385 mm and the blow ratio 1:1.2. Each of thetubular films is cut helically under an angle of 30° and four suchfilms, each having a width of about 1250 mm, are laminated and stretchedwith the surface layers facing one another in the following sequence:

(1) lamination layer to lamination layer,

(2) release/seal layer to release/seal layer,

(3) lamination layer to lamination layer.

Initially, the assembly of the four films, which still are not bondedtogether, is preheated on a pair of rollers to about 50° C. and fedunder tension into the apparatus shown in FIGS. 6A and 6B as describedfurther below.

The apparatus of FIGS. 6A and 6B comprises a long grooved roller 9 andmutually staggered rows of short grooved rollers 10 and 11 on oppositesides of the roller 9. Each of those rollers has a groove profile asshown in FIG. 7. Each groove comprises a base 6a or 6b, outwardlyinclined side walls 5a or 5b and a peak 7a or 7b. The side walls 5a and5b of opposing rollers are parallel over a portion of their depth. Eachbase 6a and 6b is of a size such that the film is not under compressionbetween the peak and base, even though it is under compression betweenthe side walls 5a and 5b. This is achieved by shaping each base so as toprovide a small space 8 between the film and the base. The peaks 7a onone roller are preferably separated by about 1 mm, in the actual example1.4 mm. The angle between the parallel portions of the side walls is 55°and the radius of curvature on the tips 0.20 mm. The rollers 10 and 11are mounted so that they can be pressed against the roller 9 with anychosen force. In practice the roller 9 is driven (by means not shown)and the rollers 10 and 11 can then be driven by the roller 9 through thesheet 4.

The apparatus includes a longitudinal stretching unit consisting of foursmooth rollers 17, 18, 19 and 20 (19 and 20 forming a nip) driven at aspeed so as to give the desired degree of stretching, and it alsoincludes at least two further pairs of intermeshing grooved rollers 21,22 and 24, 25. These are of the design shown in FIG. 8 and are alldriven rollers.

Conventional intermeshing rollers A and B for the transverse stretchingof a film 4 are shown in FIG. 8 and have circular grooves formed of abase 3a, 3b side walls 2a, 2b and peaks 1a and 1b. The peaks on oneroller have a separation of about 4 mm.

After the first set of conventional grooved rollers 21,22 thecross-sandwich is passed over banana roller 23 which mildly tenters thecross-sandwich and spreads it by eliminating the pleats formed by thegrooved rollers 21 and 22. The film is then passed through the secondset of conventional grooved rollers 24, 25 to form a pleated shape and,without further tentering, through heated rollers at 80° C. 26, 27 which"iron" the pleated cross-sandwich. The rollers 21, 22, 24 and 25 providethe transverse stretch to the laminate. The film must be relatively coldwhilst progressing through those rollers or the memory of the increasedthickness portions will not be retained.

Therefore an air jet of ambient temperature is blown through the nip ofthe grooved rollers 21, 22 and 24, 25 on both sides of thecross-sandwich to cool the sandwich and remove stretching heat, therebyalso minimising the tendency to "necking". The ambient temperature is33° C. (the trial forming the basis of this example having been carriedout in a tropical country).

The long roller 9 is formed of 25 segments of hardened steel each 60 mmlong and the two rows of short rollers, each comprising 13 rollers, arealso formed of hardened steel and each matches a segment. Each segmentterminates at each end in a half-groove which is about 0.05 mm widerthan the half width of the other grooves, so as to allow forinaccuracies in assembling the rollers and at the same time preventnarrow regions of the film sandwich from becoming strongly squeezed byend teeth on the rollers 10 as well as be end teeth on rollers 11. Thesegments are firmly screwed together axially on a common core. Thesegments have a diameter of 200 mm and the short rollers a diameter of150 mm. The film speed entering the rollers is about 25 m/min.

The roller pressure on the grooved rollers 9, 10, 11 is adjusted to avalue which is as high as possible without getting holes in thecross-sandwich, in actual fact to 200 kilo per cm length.

As already mentioned the cross-sandwich is preheated to 50° C. Therollers for "Lateral Calendering" (9, 10, 11) and for the subsequentlongitudinal stretching (17 to 20) are kept at 40° C.

The longitudinal stretching is set to give a stretch ratio 1.25:1 in thefinal product.

The intermeshing of the first set of grooved rollers (21, 22) used afterthe longitudinal stretching is set to give the final transverse stretchratio 1.33:1. The final area stretch ratio therefore is1.25×1.33=1.66:1. Since the original film gauge is 62 gm⁻² and the finalfilm is two-ply, this means the final gauge should be (2×62):1.66=75gm⁻². The final gauge is also directly verified as mentioned below.

Thus, like in the mentioned example 3 of WO-A-88-05378, thecross-sandwich leaving the first set of grooved rollers (21, 22) afterthe "Lateral Calendering" (9, 10, 11) and longitudinal stretching (17 to20) is mildly tentered over banana roller 23 before entering the secondset of conventional grooved rollers, 24, 25. The distance from edge toedge of the cross-sandwich is kept constant after the passage throughthe second set of grooved rollers 24, 25, but unlike the conditions inthe said example, the second longitudinal stretching is omitted, and thepleated material goes directly to the heated rollers 26, 27 for the heattreatment. These rollers are heated to 80° C., and at the end of thistreatment, the material has practically reached this temperature.

The longitudinal tension between the last set of grooved rollers 24, 25and the first heated roller 26, is kept high, whereby the pleatsintroduced in the material by these grooved rollers can be kept veryevenly distributed, but at the entrance to the heating treatment, thelongitudinal tension is reduced to a practical minimum, (i.e. beyond thenip between rollers 26 and 28) so that friction between the heatedrollers and the material is kept low to allow elimination of the pleatsby transverse shrinkage. Also, a high tension here would cause a toostrong ironing effect. The low tension further allows the material toshrink in the longitudinal direction, thereby stabilising thisdimension. In order to enable the mentioned change of tensions, roller26 is driven at controlled velocity and forms a nip together with rubberroller 28. 27 and 28 are idle rollers.

Between the heat-treatment unit 26, 27 and cooling unit rollers 29 and30 there is installed a very mildly 5 acting banana roller 31 forspreading. 29 and its counter-nip roller 32 are driven, while 30 isidling. The tensions along the line are controlled bytension-control-rollers, 33, 34, 35 and 36

The intermeshing between the last pair of grooved rollers 24 and 25 isvery carefully adjusted so that there still are some fine pleats in thematerial as it leaves the last roller of the heat treatment, but nopleats after the mild banana roller action.

Like in the mentioned example 3, the material is separated into twocross-laminates by peeling apart between the second and third films.

The cross-section is examined in microscope, and the self-supportingcapability judged by bending over an edge, with a strip acted by its ownweight.

Different samples of ordinary LDPE-film are similarly tried forcomparison.

The cross-section is of the type shown in FIG. 1, with the angle v(reference to FIG. 3) typically being 30° and the thickness at themiddle of the ribs is about 1.5 times the average thickness between theribs.

The self-supporting capability according to the mentioned testcorresponds to that of a 140 gm⁻² LDPE-film, while the gauge of thecross-laminate is found to be 73 g m⁻² which agrees closely with thecalculated value mentioned above.

EXAMPLE 2

One objective of this example is to demonstrate the 2nd aspect of theinvention, in which a sheet in two steps is transversely stretched andformed between compressionally working grooved rollers in a "registered"system with longitudinal stretching between these two compressionalstretching and forming steps.

Another objective of the example is to demonstrate a particularefficient procedure leading to the cross sectional configuration shownin FIG. 2, i.e. the configuration in which the convex side of the ribs,in alternating arrangement, is on one and on the other side of the crosslaminate (contrary to the arrangement resulting from the procedure ofexample 1).

The process follows the flow sheet of FIG. 5, except that there isapplied a 2nd longitudinal stretching step between steps 6 and 7.

The composition of co-extruded film, extrusion conditions, conditions ofspiral cutting and arrangement of 4 spiral-cut films for lamination allare as described in example 1, except that the gauge of each of theco-extruded films in one trial is 65 gm⁻² and in another trial 130 gm⁻².Thus, the sandwich used in the process consists of 4 films, each with a30° angle between the longitudinal film direction (the machinedirection) and the direction of melt-orientation, and with thedirections of melt-orientation arranged such that the 2-ply laminatewhich come out at the end of the line both are cross laminates.

The combined stretching, lamination and forming of "U-Ribs" is carriedout in the line shown in FIG. 6A, except that the apparatus indicated bya box now is the roller-line which schematically is shown in FIG. 9.

General description of the stretching line and--process:

The first step of compressionally transverse stretching and formingtakes place between, on one side of the film-sandwich the grooved roller37, and on the other side the 2 mutually staggered rows of short groovedrollers 38 and 39. The similar 2nd compressional step takes placebetween the grooved roller 40 and the 2 rows of short rollers 41 and 42.

Each of these sets of apparatus for compressional stretching and formingconsists of (see FIG. 6B) roller 9, staggered rows of short rollers 10and 11 and connected equipment 12, 13, 14 and 15, and the surfacepattern on the grooved rollers corresponds to that shown in FIG. 7.Dimensions of the rollers and of the surface pattern are mentionedbelow.

In order to withstand the high forces of the compression, each of thesegrooved rollers is made of hardened steel. The surface pattern ismachined with an accuracy of ±5 micron.

The 4-ply film-sandwich comes from a roller unit (not shown) in which itis heated to 60° C. It is taken-up by the smooth nip-rollers 43/44before it meets the grooved rollers 37, 38, 39 for first transversestretching and forming. From 37 it passes into the first longitudinalstretching apparatus consisting of rollers 45, 46, 47 and 48 and thenmeets the rollers 40, 41 and 42 for 2nd transverse stretching andforming. In order to "register" the two forming steps, the surfaces ofrollers 45, 46 and 47 have guiding tracks as shown in FIGS. 10 and 11.Further about this guiding will be mentioned later. The mainlongitudinal stretching in this part of the machine takes place betweenrollers 45 and 46.

The stretched sandwich then passes into the 2nd longitudinal stretchingapparatus consisting of the smooth rollers 49 to 55 and continuesfurther as shown in the flow sheet FIG. 5, or in other words through thesteps which in FIG. 5 are numbered 7 to 12. As it appears from this, thecross laminate of 4 films is split into two biaxially stretched 2-plylaminates at the end of the entire process line.

All rollers are supplied with drives, except the rollers in thestaggered rows 38/39 and 41/42, and except rollers 44, 48 and 55 (whichare driven only from the counter rollers through the film-sandwich). 44,48 and 55 are rubber coated rollers used to form a nip, while all otherrollers are steel rollers and have an internal water-circulation eitherfor maintenance of the temperature of the film-sandwich or for cooling,as the case may be (see below).

Temperature:

As mentioned, the sandwich which is fed between the nip rollers 43 and44 already has been heated to 60° C. in apparatus not shown. Roller 43is also heated to the same temperature. The compressionally workinggrooved rollers 37, 38 and 39 ought to be maintained at a slightly lowertemperature than the film-sandwich. Should they by mistake get a highertemperature there will be a risk that "necking" will occur in the middleof the ribs as shown in FIG. 4, or deeper than shown. Therefore roller43 and the bearings and housing in which the short rollers 38 and 39 arenested are kept at 50° C. and there is constantly blown ambient air on38 and 39. Ambient temperature about 20° C. After the 2nd transversestretching and forming process, the laminate is cooled to about 20° C.before any essential further stretching, and the laminate is kept atabout this temperature at all steps before the heat treatment. Thereforerollers 49 to 54 in FIG. 9 and 21, 22, 24, 25, 29 and 30 are keptcontrolled at the temperature 20° C.

The main longitudinal stretching during the 2nd step takes place between52 and 53, whereby the laminate can be sufficiently cooled by thepassage over rollers 49 to 52. Like in Example 1, the temperature of theheat treatment rollers (29 and 30, see FIG. 6A) is kept at 80° C.

Velocities of rollers, stretching ratios

When velocities of rollers are mentioned in the text below, this refersto the circumferential velocities.

The velocity at the end of the total stretching line, i.e. afterseparation into the 2-plies cross laminates, is set to 60 m/min (1 ms⁻).

In order to avoid wrinkles, the roller 37 runs 5% faster than roller 48,and in order to secure a precise transfer to roller 45, this moves 5%faster than roller 37. The ratio of velocities between rollers 45 and46, which establishes the main longitudinal stretching between the twoforming steps, is variable. The adjustment of this will be mentionedlater. Roller 47 moves at the same velocity as 46.

In order to produce optimum strength properties, the longitudinallystretched laminate is preferably relaxed, at least in part, betweenrollers 47 and 40. Therefore the ratio between the velocities of theserollers is also variable. It is adjusted subjectively to a value whichgives about minimum tension in the laminate without creation of anywrinkles.

The smooth longitudinal stretching rollers 49, 50, 51 and 52 mutuallymove at the same velocity, which is 5% faster than that of 40. The ratiobetween 52 and 53 again is variable (about the adjustment: see below).54 and 55 move at the same velocity as 53. The setting of tensionsduring the rest of the stretching line (see FIG. 6A) corresponds to whatis written in Example 1.

The intended longitudinal stretch ratio in the final product is 1.35:1.This is achieved by trial and error, while varying the ratios ofvelocities between rollers 45/46 and 52/53, at the same time keepingthese two ratios equal.

The intermeshing of the first set of grooved rollers (21, 22) used afterthe full longitudinal stretching is set to give the final transversestretch ratio 1.35:1, i.e. the same as the longitudinal stretch ratio.The final area stretch ratio therefore is 1.35×1.35=1.82:1. Thiscorresponds to a final gauge after separation into two 2-plies, of inone trial 65×2:1.82=71 gm⁻² and in the other trial 1.30×2:1.82=143 gm⁻²,which also is established directly. Like in Example 1, the intermeshingbetween the last pair of grooved rollers (24, 25) is very carefullyadjusted so that there still are some fine pleats in the material as itleaves the last roller of the heat treatment, but no pleats after themild action of banana roller 31.

Pattern of roller surfaces, distance between rollers, diameters ofrollers, roller pressures

As mentioned above, the surfaces of rollers 37, 38, 39, 40, 41 and 42are formed as shown in principle in FIG. 7. More specifically, the anglebetween the parallel portions of the surfaces of the grooves is 55° andthe division measured from middle to middle of each tip is 1.60 mm, i.e.0.2 mm bigger than that used in Example 1. The reason for this is thatthe machine must be able to stretch a heavier sandwich, namely in thesecond trials 4×135 gm⁻² which is equivalent to about 600 micronthickness.

The radius of curvature on the tips are: on rollers 37, 38, 39 0.20 mmand on rollers 40, 41, 42 0.30 mm. Under the prevailing circumstances,the 0.2 mm has been found to be about the smallest which can be appliedwithout creating a neck as shown in FIG. 4, and it leads to a veryefficient waving of the cross section of material, with the result thatthe laminate easily "falls into track" on the following rollers.

The reason why the radius of curvature on the tips on the rollers 40,41, 42 is bigger than this, namely 0.30 mm, is that the squeezed,band-formed portions in the second forming process then become narrowerthan the squeezed band-formed portions in the first forming process,which has the effect that the thickening of the ribs becomes morepronounced. If the stretching line were destined only for manufacture ofcross laminates without the "U-Ribs" (as the final products in Example3) it would have been more convenient also to use 0.20 mm as radius ofcurvature on the tips of rollers 40, 41, 42.

Rollers 45, 46 and 47, which have the function to transfer andlongitudinally stretch the laminate without bringing the wavedconfiguration out of order, have much more shallow grooves (tracks) withthe same division as those on the grooved rollers for compressionallyforming and stretching. On roller 45, the angle between the roller axisand the surfaces of the grooves is 45°, by which they approximately fitwith the configuration of the waved cross-sandwich (see FIG. 10) whilethe corresponding angle on rollers 46 and 47 is only 30° (see FIG. 11)in order to permit the laminate a transverse contraction to which itnaturally tends during the longitudinal stretching.

The statement above that the grooves (tracks) on the longitudinalstretching rollers have the same division as the grooved rollers forcompressional stretching and forming, must be understood on an averagebasis as follows: like in Example 1, the long grooved rollers are formedof segments (in this case each segment is 80 mm long) screwed togetheron a common core, and each segment terminates at each end in ahalf-groove which is 0.05 mm wider than the half-width of the othergrooves. (Explanation: see Example 1). This extra 0.05 mm at each end ofeach segment is taken into consideration in calculation of the averagedivision, so that errors don't add up from roller to roller.

The waved laminate exhibits a strong tendency to expand laterally beforethe longitudinal stretching, while the longitudinal stretching creates atendency to contract laterally. Under the stretching conditions selectedin this example, the tendency to expand is prevailing, under otherconditions it can be oppositely. However, the tracks on the rollers havethe function to avoid any expansion or contraction. In order to achievethis effect, the distance between rollers 37/45, 45/46, 46/47 and 47/40are only a few mm each and are adjustable. These rollers are veryprecisely lined-up to keep the waved laminate properly "in track". Tothis end, the frames for the rollers is made sufficiently stiff, andthere is chosen a relatively big diameter (300 mm) for all rollers whichotherwise by bending could disturb the alignment. The other drivenrollers have the same diameter (300 mm) but in this case in order to getsufficient heating/cooling surfaces.

The distance between 43/37 and between each pair of neighbour rollers inthe row 40-54 are less critical and are chosen to about 20-50 mm each.The bearings and frame work is arranged in such a way that the rollerscan be brought apart during threading of the line.

The short grooved rollers In rows 38/39 and 41/42 have diameter 150 mmas in Example 1. The pressure between these rollers and theircorresponding long rollers 37 and 40, respectively, is set at thehighest value which can be applied without making holes in thecross-sandwich. With feeding of 4×65=230 gm⁻² film-sandwich, a rollerpressure of 200 kg per cm roller length is chosen, and with feeding of4×130=520 gm⁻² film-sandwich a roller pressure of 300 kg per cm rollerlength.

Results

Samples from the 71 gm⁻² cross-laminate is tested in the same way as inExample 1, while samples from the 143 gm⁻² cross-laminate only is testedby the microscopical examinations. Both sets of samples show the convexside of the ribs in alternating arrangement on one and on the other sideof the laminate as shown in FIG. 2. The gauge between the ribs isclearly more even than observed on the laminate of Example 1. The anglev (reference to FIG. 3) typically is about 40° on one side and 30° onthe other side, and the thickness at the middle of the ribs is about 1.8times the average thickness of the material between the ribs.

The self-supporting capability according to the test mentioned inExample 1 for the cross-laminate of gauge 71 gm⁻² corresponds to that ofa 160 gm⁻² LDPE film. Compared to the simpler and cheaper process lineused in Example 1, the process used in Example 2 has the followingadvantages:

1. It can make the convex side of the ribs alternately on one and on theother side (FIG. 2) even when the material is separated in the middle atthe end of the process.

2. It can produce the "U-Rib" structure at higher linear velocities.

3. It can operate with thicker material.

4. The material between the ribs becomes more even.

EXAMPLE 3

The purpose of this example is to show how the procedure of Example 2can be modified to make a cross-laminate without "U-Ribs", when this isdesirable, and in this connection demonstrate the advantages of carryingout the two compressional transverse stretching steps in "registered"manner.

The procedure of Example 2 is repeated in exactly the same way except asfollows:

1. Longitudinal and transverse stretching ratio as measured at the endof the process each is 1.40:1. Areas stretch ratio therefore1.40×1.40=1.96:1.

2. Temperature of heat treatment 100° C. (Rollers 26 and 27).

3. The intermeshing between the last set of grooved rollers is adjustedby trial and error to a value slightly lower than that which restoresthe U-shape. (If much lower, the thickness will become more irregular).

The process is carried out both with the four cross-sandwiched films ofeach 65 gm⁻² and those of each 130 gm⁻².

For comparison, similar trials are carried out without "registration",and for this purpose the line is operated in two steps. After preheatingof the 4-ply, both steps start between rollers 47 and 48. The first stepends with roller 55, after which the 4-ply is spooled up for use againin second step.

In this step, the 4-ply goes through the entire route from 47/48 to andincluding the heat treatment, cooling and separation.

Before first step, the 4-ply is heated to 60° C. and rollers 47/48 areheated to the same temperature, while all following rollers are kept at50° C. Before second step, the 4-ply is heated to 50° C. and the rollersfor compressional transverse stretching are kept at this temperaturewhile the rollers for longitudinal stretching are kept at 20° C.

The subsequent processes are carried out under the same conditions asexplained above in connection with the "registered" procedure.

In both steps, rollers 47/48 are used to give the 4-ply a 5% strainbefore the compressional stretching. Total stretch ratios are the sameas in the "registered" process.

Results:

Cross sections of all of the final products are examined by microscope.Those which were stretched in "registered" manner exhibit the thickerribs at even spacing, but between the ribs the thickness is very even. Amild cold calendering easily eliminates the ribs, and the thickness thenbecomes even all over.

Those samples which were not stretched in "registered" manner, showintervals of about 10-20 mm width which look almost like the abovementioned regular structure (because of the tendency of the waved filmto "fall into track") but between each of these intervals there areintervals of about 5-10 mm width in which the structure is irregular,with spots of the cross section frequently less than half of the averagethickness.

With the available apparatus for cold calendering, it was not possibleto eliminate the thin lines of these cross laminates.

EXAMPLE 4

Example 1 and the "registered" part of example 3 are repeated with thedifference that the starting films are made on basis of polypropylene.Composition of middle layer of the co-extruded films (75% of the totalfilm): 80% homo-polypropylene of melt flow index of 0.3 (ASTM No. D-1238condition L)+20% LLDPE (same ASTM but condition E).

The surface layers, 10% and 15% of the total film are the same as inexamples 1 and 2.

Film gauges: 60 gm⁻² for the trial corresponding to Example 1 and 65 and130 gm⁻² respectively, for the two trials corresponding to Example 2.

Otherwise the procedure of these examples are exactly followed.

Results:

Cross sectional profile: The shapes of the U-Ribs closely correspond tothe respective samples of examples 1 and 2, except that the trialcorresponding to Example 1 leads to a more even thickness between theribs.

The self-supporting capability, expressed as the thickness of LLDPE filmwhich is equivalent in this respect, is about 20% higher than exhibitedby the cross-laminates of Examples 1 and 2.

I claim:
 1. An elongated cross-laminate of at least two films formedfrom orientable thermoplastic polymer material, in which each film isuniaxially oriented or biaxially oriented in an unbalanced manner tothereby have a major direction of orientation, and the major orientationdirections of the individual films criss-cross each other with the majordirection of orientation of at least one of said films extending at anangle to the length direction of the cross-laminate, and thecross-laminate exhibits a pattern of generally parallel lengthwiseextending striations constituted by thickness variations thereinresulting from stretching at a given stretch ratio of the cross-laminatetransversely to the length direction, wherein said pattern of striationscomprises elongated ribs extending in said length direction each ofwhich is substantially defined between generally concave and generallyconvex curved surfaces which are in generally coincident relationtransversely of said length direction on opposite faces of saidcross-laminate, the radius of curvature of said concave surface beinggenerally greater than that of said convex surface whereby the ribs havea maximum thickness greater than the average thickness of the laminate,and each rib as a whole has a generally shallow U-shaped transversecross-section, and the laminate material in or adjacent to lateralboundaries of the ribs in the tensionless state of the material aregenerally reversely curved relative to the rib curvature to give thematerial between adjacent pairs of ribs a generally flattened condition.2. A cross-laminate according to claim 1, wherein said pattern ofstriations comprised by said elongated ribs is a generally regularpattern over substantially the entire area of said cross-laminate.
 3. Across-laminate according to claim 2, wherein the convex surfaces of theribs alternate in series from one side to the other side of thecross-laminate.
 4. A cross-laminate according to claim 2, wherein theconvex sides of the ribs are on the same side of the cross-laminate. 5.A cross-laminate according to claim 2, wherein the average separationbetween the ribs of adjacent pairs of ribs is between 1 and 10 mm,measured from peak to peak and taken as an average.
 6. A cross-laminateaccording to claim 2, wherein the average thickness of the generallyflattened material between lateral rib boundaries, a boundary beingdefined as the locus where the rib thickness equals the averagethickness over a local region including several neighboring ribs and thegenerally flattened material therebetween, is at least 15% andpreferably at least 30% less than the maximum thickness of the rib.
 7. Across-laminate according to claim 1, wherein the U-shaped cross-sectionof each said rib has a generally re-entrantly curved base between spacedapart sides and planes drawn tangentially to said sides intersect togenerally define therebetween an included angle of about 10 up to about90.
 8. A cross-laminate according to claim 1, wherein the longitudinallyextending U-shaped ribs are locally interrupted by a pattern oflongitudinally separated transverse deformation lines wherein theincreased thickness and U-shaped profile of the ribs are substantiallyremoved to thereby enhance the flexibility of the cross-laminate abouttransverse lines.
 9. A cross-laminate according to claim 1, wherein eachof said at least two films has three plies including a main layer forstrength in the middle and minor layers at its opposite surfaces tofacilitate bonding between the films and impart heat-seal propertiesthereto and the main layer of said films consists of 10-30% low densitypolyethylene which is mainly linear low density polyethylene, and theremainder is high-molecular weight polyethylene, high molecular weightpolypropylene or a combination of both.
 10. A cross-laminate accordingto claim 1, which also has an undulating or zig-zagging curvature as awhole when viewed in longitudinal section.
 11. In a process ofmanufacturing a continuous elongated web of rib-patterned cross-laminatefrom at least one cross-sandwich web, each such cross-sandwich web beingformed from at least two films of orientable thermoplastic polymermaterial wherein the films are each uniaxially or unbalanced biaxiallyoriented to impart thereto a major direction of orientation and at leastone of the films exhibits an angle between its longitudinal directionand its major direction of orientation, and said at least two films aresandwiched together in a criss-crossing arrangement of the respectivemajor directions of orientation, in which said cross-sandwich web isstretched in the direction transverse to its length by passage of thesame lengthwise between grooved rollers and said at least two films arelaminated together, the improvement wherein the transverse stretching iseffected by forming said cross-sandwich web while at a temperature belowthe melting point of the thermoplastic material into undulatingcross-sectional shape; during or after said undulating forming process,stabilizing in their curved shape tip portions of the undulations on atleast one side of the cross-sandwich web; and transversely stretchingthe stabilized cross-sandwich web in regions between the stabilized tipportions by the use of grooved rollers, this latter stretching beingadapted to retain in the stabilized portions either said undulatingshape or at least a memory of said undulating shape; and if saidstabilized portions retain only a memory of said undulating shape,subsequently heat-treating the cross-sandwich web so that the materialin local regions between the stabilized tip portions shrinks along ahypothetical plane lying substantially midway between the surfaces ofthe cross-sandwich web in a direction perpendicular to the direction ofmovement of the web, so that said stabilized tip portions which retainonly said memory of the undulating shape are reshaped at least in part,thereby to create a rib patterned cross-laminate web with a pattern ofelongated lengthwise ribs of generally shallow U-shaped transversecross-section, said ribs having a maximum thickness significantlygreater than the average thickness of the cross-laminate as a whole. 12.A process according to claim 11 in which the heat treatment is conductedwhilst subjecting the web to mild transverse tension.
 13. A processaccording to claim 11 in which the heat treatment is conducted byforming the cross sandwich web into pleats of predetermined size andcontacting the pleated web with a hot, smooth roller, to therebysubstantially iron out said pleats while maintaining the cross sandwichunder a transverse tension sufficiently low for said stabilized tipportions to undergo said re-shaping.
 14. A process according to claim 11wherein the cross-sandwich web is longitudinally stretched prior to orimmediately following said forming of the web into said undulating crosssection and the stabilization of the tip portions of the undulating web.15. A process according to claim 11, wherein the stabilization of thetip portions is effected by carrying out the forming of thecross-laminate web into said undulating cross-section while thecross-laminate is at a temperature close to the melting point of saidthermoplastic polymer material.
 16. A process according to claim whereinsaid thermoplastic polymer material is adapted to undergo cross-linkingunder irradiation and the stabilization of the tip portions is carriedout by cross-linking the polymer material of said web under irradiation.17. A process according to claim 11, wherein the forming of saidcross-sandwich web into undulating cross-section and stabilization ofthe tip portions thereof are carried out simultaneously by passing thecross-sandwich through compressionally working grooved rollers.
 18. Aprocess according to claim 11, wherein a plurality of saidcross-sandwich webs, each having the two polymer films arranged withtheir orientation directions in criss-crossing relation, are stackedtogether and processed simultaneously and after the processing theplural cross-sandwich webs are separated from the stack.
 19. A processaccording to claim 11, wherein as a measure to maintain the memory inthe stabilized portions, the stretching between grooved rollers of thecross-sandwich web is carried out with the material in the stabilizedportions being near room temperature, preferably between 15°-40° C. 20.A process according to claim 13, wherein during said heat treating thecross-sandwich web is allowed to undergo both an overall transversecontraction and an overall longitudinal contraction.
 21. In a processfor transversely stretching a polymer film or a plurality of polymerfilms arranged in sandwich relation by passing the same between opposedgrooved rollers each having on the periphery thereof a pattern ofcircular or helical grooves and intervening peaks with the peaks of oneroller in intermeshing relation with the grooves of the opposite roller,and in which the film or film sandwich contacts only the tips of thepeaks of the grooved rollers, the improvement of directing a flow of afluid coolant medium through the nip of the grooved rollers on at leastone side of the film or film sandwich while the same passes between saidopposed grooved rollers in order to remove stretching heat and keep thefilm at the desired temperature during stretching.
 22. A method ofbiaxially stretching an elongated web of polymeric sheet material by thesteps of:1) subjecting the web of material to a first simultaneoustransverse stretching and forming operation in which opposite surfacesof the material are compressed towards toward each other over at leastlaterally spaced longitudinal regions thereof by passing the materialthrough compressionally working opposed grooved rollers from which thematerial emerges in a transversely undulating configuration, 2)downstream of the compressionally working grooved rollers stretching theweb longitudinally by passing the same through opposed rollers rotatingat a higher circumferential rate of speed than said compressionallyworking grooved rollers, and 3) subjecting the longitudinally stretchedweb to a second transverse stretching operation by passing the samethrough additional opposed grooved rollers, wherein the stretchingconditions are selected to remove from the web material a portion ofsaid undulations while retaining at least a trace of said undulationstherein; and said second stretching operation is correlated to theundulations of said first operation by either:a) adjusting theseparation between the undulations in the web to match the separationbetween the grooves of said additional grooved rollers, or b) utilizingfor each of said first and second operations opposed grooved rollershaving substantially the same separation between the grooves thereof andlocating any intervening pair of rollers through which the web passesbetween said first and second operations sufficiently near a nextupstream pair of rollers that the separation between the undulations ofthe web remains constant.
 23. A method according to claim 22 whereinsaid additional opposed grooved rollers are also compressionally workingopposed grooved rollers compressing opposite surfaces of the web towardstoward each other over at least laterally spaced longitudinallyextending regions thereof.
 24. A method according to claim 22 wherein inthe practice of alternative b), at least one pair of feed rollers forsaid web is provided at a location intervening between saidcompressively working grooved rollers and said additional groovedrollers and each such intervening pair of rollers is provided on itsperipheral surface with a pattern of transversely spaced guiding trackshaving a transverse separation that is the same as the separationbetween the grooves of said grooved rollers.
 25. A method according toclaim 23 wherein the opposite surfaces of said web are compressed towardone another in both of said first and second simultaneous transverse andforming operations by the respective compressionally working opposedgrooved rollers in longitudinally extending transversely spaced bandscorresponding to regions of the undulations formed in the web betweenthe tips of adjacent peaks of said undulations leaving the tips of saidpeaks free of such compression, and the transverse dimension of thebands of web subjected to compressional working by said first groovedrollers is larger than the transverse dimension of the bands subjectedto compressional working by said second grooved rollers.
 26. A methodaccording to claim 22 wherein said web is subjected to a furtherlongitudinal and/or transverse stretching operation subsequent to thesecond stretch in operation.
 27. A method according to claim 26 whereinsaid web is heated to a temperature above room temperature before beingsubjected to said first simultaneous transverse stretching and formingoperation and is maintained at the heated temperature until after saidsecond transverse stretching and forming operation and is cooled to roomtemperature before being subjected to said further stretching operation.28. A method according to claim 22 wherein as a final step, the web ispassed through at least one pair of smooth-surfaced calendaring rollersto calendar the web.
 29. A bag made from a cross-laminate according toclaim
 1. 30. Apparatus for biaxially stretching an elongated film ofpolymeric material which comprises in sequence:1) A first transversestretching and film forming station comprising a first set of drivenopposed grooved rollers each having on the periphery thereof a patternof circular or helical grooves and intervening peaks with the peaks ofone roller in intermeshing relation with the grooves of the oppositeroller, the grooved rollers of said first set being urged together tocompressionally work the film passing therebetween, to therebytransversely stretch the film and simultaneously form the same uponemergence from the first roller set into a transversely undulatingconfiguration; 2) A stretching station to stretch the undulating filmlengthwise and partially but not completely remove the undulationstherefrom; 3) A second transverse stretching and forming stationcomprising a second set of driven grooved rollers each having on theperiphery thereof a pattern of circular or helical grooves andintervening peaks with the peaks of one roller in intermeshing relationwith the grooves of the opposite roller, to thereby transversely stretchthe film and simultaneously form the same upon emergence from the secondroller set into a transversely undulating configuration; and 4)registration means for maintaining the undulations in the film emergingfrom the second roller set in substantial transverse registration withthe undulations in the film emerging from the first roller set.
 31. Theapparatus of claim 30 wherein the axial spacing of the grooves of bothsaid first roller set and said second roller set are substantially thesame and said registration means comprises support means for therespective first and second roller sets to support the same insufficiently close proximity that the axial spacing of the undulationsin the film emerging from said first roller set is preserved until thefilm reaches the second roller set.
 32. The apparatus of claim 30wherein said registration means comprises at least one registrationroller arranged between said first and said second sets of rollers, eachsaid one registration roller having on its periphery a pattern ofcircumferential guiding tracks for aiding in bringing the undulations ofthe film into registration with the grooves of the second set of groovedrollers.
 33. The apparatus of claim 32 wherein said guiding tracks arein the form of generally V-shaped circumferential depressions in theroller periphery.
 34. Apparatus according to claim 30, in which,downstream of the second set of grooved rollers, there is at least onefurther transverse and/or longitudinal stretching station.
 35. Apparatusaccording to claim 34 which comprises a heat treatment station aftersaid longitudinal or transverse stretching station.
 36. Apparatusaccording to claim 35 in which the heat treatment station comprises asmooth heated roller and means for applying controlled tension to thefilm as it contacts the heated roller, and immediately upstream of theheat treatment station there are grooved pleating rollers to providecontrolled pleating of the film, by contacting the film at the tips onlyof the groove pattern of the rollers.
 37. Apparatus according to claim36 comprising cooling means for the film as it passes through thegrooved pleating rollers, said cooling means comprising means forsupplying sa flow of a cooling medium to the nip of pleating rollers onat least one side of the film.
 38. Apparatus according to claim 30wherein the compressionally working first set of opposed grooved rollersexerts a compressional force against opposite surfaces of said filmpassing therebetween along longitudinally extending transverselyseparated regions of said film.
 39. Apparatus according to claim 38wherein said second set of opposed grooved rollers is alsocompressionally working to exert compressional force against oppositesurfaces of the film passing therebetween along longitudinally extendingtransversely separated regions of the film and the transverse dimensionof the regions worked by the first set of opposed grooved rollers isgreater than the transverse dimension of the regions worked by thesecond set of opposed grooved rollers.
 40. Apparatus according to claim30 wherein the grooves of at least said first set of opposed groovedrollers each comprises a base and on each side of the base, upwardlyinclined side walls and a peak which is shared with any adjacent groove,the side walls of opposed intermeshing grooves on the opposite rollersbeing generally parallel over at least a portion of their length, thegenerally parallel portions of said side walls of opposed groovescompressing therebetween longitudinally extending regions of oppositesurfaces of said film.
 41. Apparatus for transversely stretching a filmof polymer material which comprises a pair of opposed grooved rollersthrough which the film is adapted to be passed, each roller havingtherealong alternating circular or helical grooves and peaks, means forbiasing said opposed rollers together to bring opposite grooves andpeaks into intermeshing relation with the film passing therebetween incontact only with the tips of said peaks of the opposed rollers to formthe film into a transversely undulating configuration to therebytransversely stretch the film, and cooling means for cooling the filmwhile undergoing such stretching comprising means for a directing a flowof coolant medium through the nip of the opposed rollers on at least oneside of the film.
 42. Bag made according to the process of claim 11.